Project Summary Studies have repeatedly shown that breast density, which limits mammographic sensitivity, is also a strong risk factor for breast cancer. As a result, breast density information is increasingly utilized in guiding personalizing breast cancer screening and prevention. Conventional 2D mammography, however, is inherently limited due to the effect of tissue superimposition in estimating breast density. In addition, the commonly used measures of mammographic density cannot fully capture the heterogeneity of the breast parenchymal pattern, shown to be an important additional indicator of breast cancer risk. Breast Tomosynthesis is an emerging 3D x-ray modality which offers superior, tomographic breast tissue visualization compared to 2D mammography. Multiple studies have shown that screening with tomosynthesis reduces recalls while increasing cancer detection compared to screening with mammography alone, which is currently fueling a broad implementation of tomosynthesis for general population breast cancer screening. In addition to improved screening performance, our hypothesis is that 3D measures of breast density and parenchymal pattern complexity from tomosynthesis can outperform density measures from conventional 2D mammography, to improve breast cancer risk estimation. During the previous phase of this award, we developed innovative methods for breast density and parenchymal texture analysis in digital mammography and evaluated their association to breast cancer risk. Our studies have shown compelling evidence that these measures provide powerful new imaging markers that can augment the standard mammographic measures. This renewal application focuses on extending our work to the emerging technology of tomosynthesis, by developing novel measures of breast tissue composition from tomosynthesis and by determining their association to breast cancer risk. Towards this end, we will perform the largest association study reported to-date for tomosynthesis and breast cancer risk, including a well-characterized and diverse sample of 675 cases and 2700 controls nested within two large academic breast cancer screening practices at the University of Pennsylvania (UPenn) and the Mayo Clinic. In AIM1 we will extend and optimize our mammographic parenchymal analysis software for breast tomosynthesis using sophisticated computational methods pioneered from our group for breast tomosynthesis; in AIM2 we will determine associations between breast cancer and the novel tomosynthesis density and texture measures, using retrospective data analysis as our training set; and in AIM3 we will perform independent validation using prospectively collected, ethnically diverse, samples from both institutions. Within this setting, we will also evaluate the performance of measures derived from synthetic digital mammograms, increasingly replacing conventional digital mammography images in tomosynthesis acquisition. This study will be the first to evaluate tomosynthesis in cancer risk assessment and will develop the necessary tools to enable larger multi-center studies. Ultimately, these novel biomarkers could lead to more accurate risk prediction, improving personalized breast cancer screening and prevention.